FR2681718A1 - DEVICE FOR COOLING THE HEART AND FOR PROTECTING THE CONCRETE STRUCTURE OF A NUCLEAR REACTOR WHOSE CORE IS FUSED AFTER AN ACCIDENT. - Google Patents

Abstract

The reactor comprises a vessel (3) enclosing the core (4) of the reactor arranged with its vertical axis in a vessel shaft (2). The cooling and protection device (11) rests on the bottom of the reactor pit. It consists of a metal hearth (11) covering the bottom of the vessel shaft (5) in which cooling channels (20) are formed by circulating a cooling fluid. Means (12, 13, 14) for supplying and discharging the cooling fluid are connected to the cooling channels (20).

Description

The invention relates to a cooling device.

  dissement of the core and protection of the concrete structure of a nuclear reactor whose core has entered

merger following an accident.

  Pressurized water nuclear reactors

  have a generally cylindrical tank containing

  mant the reactor core, arranged with its vertical axis in a cylindrical vessel well having a bottom bottom

  located directly above the vessel The core of the nuclear reactor

  is cooled by pressurized water circulating in the primary circuit of the reactor and inside

  the tank in contact with the fuel assemblies.

  In the case of certain accidents occurring in the nuclear reactor and resulting in a loss of the cooling function of the core, we must consider,

  given the very serious consequences that result-

  although, the probability of such an event is extremely low and practically zero, the case where the emergency injection circuits of the reactor could not be put into operation An accidental sequence can then occur leading to the melting of the core in the absence of cooling water, which can lead to

  destruction by piercing the bottom of the tank and the

  lement of the mass of the molten core and of the materials surrounding the core, in the concrete well containing the reactor vessel The contact of the molten mass of the fuel and of the materials surrounding the fuel, called corium, the temperature of which can reach 2500 at

  28000 C, with the bottom of the concrete tank well below

  sence of cooling, can lead to the complete destruction of the bottom of the well The corium can then sink into the slab of the reactor containment, destroy this slab and contaminate the groundwater present in the soil of the nuclear reactor site The

  progress of the corium inside the soil cannot stop

  ter only when the residual power of the corium has decreased sufficiently. Various devices have been proposed to avoid contact of the corium with the bottom of the concrete tank well. Known devices generally make it possible to spread the mass of the corium over a certain surface, so that the power to be evacuated per unit of surface is the

  as low as possible and is compatible with the possibilities

  Fluid cooling units For example, it has been proposed to recover and contain the corium in a metal bag coated internally with materials.

  refractories whose partial fusion absorbs energy

  transiently, and makes it possible to have a sufficient time interval to externally drown the metal pocket in a body of water, in order to remove the residual power of the corium by boiling the body of water. The drawback of this device comes from the fact that refractory materials are most often very poor conductors of heat, which has the effect of increasing the equilibrium temperature of the corium which

remains in the liquid state.

  Other devices are known which use refractory soles permanently cooled by a water circuit. One of the drawbacks of these devices is that the cooling circuit can exhibit failures which are liable to make it at least partially ineffective. , the heat exchanges are not intense enough to prevent the corium remaining at a high temperature and in the state

  liquid after spillage on the recovery device

ration and cooling.

  There is also known a device consisting of a stack of profiles placed horizontally in the bottom of the well, below the bottom of the tank, so as to form receptacles for molten corium, in order to achieve the dispersion of the molten mass, d '' improve its cooling and allow its solidification This device has the disadvantage of not ensuring effective protection of the concrete of the well of tank, when the flow of corium occurs in a localized way

  profiles which are staggered are then suscep-

  tibles to fill successively with molten corium by local overflow, so that the molten mass can

  quickly reach the bottom of the tank well.

  Finally, by French patent application 91-06047, a device is known for recovering and cooling the core of a molten nuclear reactor which makes it possible to avoid any contact between the mass of the core

  molten and the concrete of the tank well and ensure a cooling

  quickening and solidification of the melt.

  This device consists of a metal structure resting on the bottom of the tank well and immersed in a body of water filling the lower part of the tank well. The metal structure has a central chimney, a recovery and cooling wall.

  of the molten core and a peripheral wall.

  This device, which makes it possible to quickly spread, cool and solidify the corium, however has the drawback that the metal structure can be destroyed, under the effect of the dynamic forces generated during the fall of the corium and the bottom of the tank, these can reach several thousand

tonnes.

  In addition, the cooling of the corium leads to the formation of an extremely large vapor flow rate, inside the vessel well, this flow rate being of the order of 10,000 m 3 / hour. The evacuation of such a flow rate of vapor in the atmosphere of the containment causes dispersion of fission products throughout the volume of the reactor building, which is incompatible

  with the security principles that must be respected.

  The object of the invention is therefore to propose a device for cooling the core and for protecting the concrete structure of a nuclear reactor, the core of which has melted following an accident, the

  reactor comprising a generally cylindrical tank

  plate containing the reactor core arranged with its vertical axis in a vessel well having a bottom bottom on which the cooling and protection device constituted by a part of the structure of the

  reactor comprising part of a slab located at the a-

  lead from the tank, this device having a very high resistance and a very high rigidity, so as to support the fall of the corium and of the tank bottom and being able to ensure the cooling of the corium without rejection of vapor

in the reactor building.

  For this purpose, the device according to the invention consists of a metal hearth covering the bottom of the tank well in which are formed cooling channels by circulation of a fluid, connected to

  means for supplying and discharging the coolant

  dissement. In order to clearly understand the invention, we will now describe, by way of nonlimiting example, by

  referring to the attached figures, a mode of realization

  sation of a cooling and protection device according to the invention used in the case of a reactor

  nuclear cooled by pressurized water.

  Figure 1 is a sectional view through a vertical plane of the safety enclosure of a nuclear reactor

  pressurized water with a cooling device

  ment and protection according to the invention.

  Figure 2 is a schematic view of the part

  bottom of the nuclear reactor vessel well depicted

felt in figure 1.

  FIG. 3 is a top view of a metal hearth constituting a cooling device and

protection according to the invention.

  Figure 4 is a side view along 4 of the

figure 3.

  Figure 5 is a sectional view along 5-5 of

Figure 3.

  Figure 6 is a sectional view along 6-6 of

Figure 5.

  Figure 7 is a sectional view of the metal sole made according to a variant showing a channel

  and its means of supply and evacuation

cuation of cooling water.

  Figure 8 is an enlarged sectional view showing a check valve of the cooling channel

shown in figure 7.

  In Figure 1, we see the safety enclosure 1 with a double jacket of a nuclear reactor inside which is disposed the vessel 3 enclosing the core 4 of the reactor, in a vertical arrangement and following

the axis of a tank well 2.

  The lower part of the safety enclosure 1 made of double-walled concrete consists of a raft resting on the ground.

  The cooling and protection device

  tion according to the invention, generally designated by the reference 10, is constituted by a metal sole ll resting on a part of the raised raft 5 and placed directly above the tank well 2, below the bottom of the tank 3 enclosing the core 4 Inside the hearth 11 are formed cooling channels which are supplied with a cooling fluid such as water by a cooling circuit comprising pipes 12

  and 13 passing through the wall of the safety enclosure 1.

  The cooling circuit consists of two half-circuits, each comprising pipes such as 12 and 13 connected to the cooled hearth 11, a heat exchanger 14 cooled by air (arrow 15), a reservoir 16 for expansion and condensate collection from the exchanger 14 and a circulation pump 17. For each of the half-circuits, the water leaving the bottom through the pipe 13 is cooled inside the heat exchanger 14 before d '' be returned to the floor by line 12, by the driving height resulting from the difference in the specific volumes of the water-vapor emulsion present in the line 13 and water

  cooled returning to the floor via line 12.

  The operation of the cooling device

  ment and protection according to the invention, in the case of a melting of the core 4 and the tank bottom will be described

  in more detail later in the text In a schematic way

  tick, if the core 4 of the nuclear reactor accidentally melts and causes the bottom of the tank to melt, the mixture constituted by the core, the cladding material and certain elements of the reactor tank, called corium, spreads over sole 11 Sole Il, which is very thick, is capable of collecting molten corium without being destroyed, ensuring the protection of

  write off 5 and cool the corium.

  Sole 11 is cooled by water circulation

  reaching via supply lines such as 12.

  The heated water or the steam is collected by the line 13 at the outlet of the hearth 11 and cooled and condensed inside the exchanger 14 The cooled water is returned to the cooling channels of the hearth

11 through line 12.

  The cooling of the corium is therefore carried out without emission of vapor in the enclosure and by using cooling means arranged outside of

  the reactor safety enclosure.

  The heat exchangers 14 and the tanks 16 of the cooling circuit are installed at a height above the horizontal median plane of the floor 11 which can be of the order of 25 m and which makes it possible to obtain

  static pressure of the water column in the channels

  supply stations, of the same order of magnitude as the pressure drop in the cooling channels of the

sole 11.

  In Figure 2, there is shown the part 5a of the raft 5 arranged projecting upwards in line with the tank well 2 and the cooled bottom 11 resting on the

part 5 has to write it off.

  A sheet metal wall 18 or skin which covers the upper surface of the raft 5 and on which a layer of protective concrete is poured is connected to

  the outer part of the metal sole 11.

  We will now describe in more detail the metal sole 11 of the cooling and

  protection according to the invention, with reference to the set

ble of Figures 3, 4, 5 and 6.

  The sole 11 is constituted by a metal plate-

  lique in which are provided cooling channels

  20 and which is connected at its longitudinal ends

  them to manifold assemblies 21 and 22 making it possible to supply the channels 20 with fluid of

  cooling and draining the coolant

  heated and possibly vaporized.

  The manifold assembly 21 is supplied with cooling water via a line 21 a and connected, in a

  opposite arrangement, to an exhaust pipe 21 b.

  Similarly, the manifold assembly 22 is connected at one of its transverse ends to a supply line 22 a and at its other end to a line

evacuation 22 b.

  As can be seen in FIG. 5, each of the manifold assemblies comprises an outer casing 24 and an inner casing 25 fixed by welding to the faces

  sides of the massive metal sole 11.

  Each of the channels 20 formed in the thickness of the sole has a pin shape and has a lower branch opening at one of its ends inside the outer casing 24 and outside the casing internal 25 of a manifold 21 (or 22) and an upper branch opening at one of its ends to

  the interior of the internal envelope 25 of one of the collectors

tors 21 (or 22).

  The successive cooling channels such as 20, 20 ′ arranged along the length of the cooled hearth 11 open out alternately in the collector

21 or in the collector 22.

  The cooling water circulates, during the operation of the device, inside each of the channels 20, between the external supply manifold 24

  and the internal evacuation collector 25.

  As can be seen in FIG. 6, the metal hearth 11 is made up of successive blocks 30 a, 31 a, 31 b ,, 31 N and 30 b welded together along their lateral faces to form successive joint planes

and parallel 32.

  The end blocks 30 a and 30 b have a height greater than the height of the intermediate blocks 31 a, 31 b, 31 N and comprise an upper part 43 projecting upwards relative to the upper part of the blocks 31, constituting a low wall intended to retain the

  mass of molten corium on sole 11.

  The end blocks 30 a and 30 b have a face intended to be attached and welded against a corresponding face of an intermediate block which is machined so as to constitute by juxtaposition with corresponding machining of the intermediate blocks, the branch

  lower and upper branch of a cooling channel

dissement 20.

  The intermediate blocks 31 a, 31 b ,, 31 N are machined on their two lateral faces which are attached and welded against a corresponding face of another intermediate block or of an end block, to form parts of the lower branches and higher by

  two successive cooling channels.

  The raised faces 43 of the end blocks

  30 a and 30 b are coated with a layer of refractory material

  shutter 35 intended to protect the low wall against corium

  hot likely to spill on the floor 11.

  The end blocks 30 a and 30 b as well as the intermediate blocks 31 a to 31 N are machined at their upper part, so as to provide, when they have been juxtaposed, parallel grooves 33, in the longitudinal direction of the sole 11 These grooves 33 make it possible to promote the transfer of heat from the corium to the cooling channels 20 and to constitute

  now traps the corium being cooled.

  These grooves also make it easier to operate

  welding of the steel blocks juxtaposed two by two.

  A mechanically welded sheet structure 34 is arranged above the upper surface of the intermediate blocks 31 of the hearth 11, so as to protect the hearth and to absorb the kinetic energy brought into play during the fall of the corium and the tank bottom on the bottom 11, in the event of an accident resulting in a core melt

of the reactor.

  The structure 34 comprises an upper part constituted by a folded sheet and a set of reinforcing plates 29 perpendicular to the upper part of the structure each engaging in a groove 33 in order to maintain the structure 34 on the sole. 11. As can be seen in FIG. 7 which relates to an alternative embodiment of the device, as regards the position of the collectors, each of the

  hairpin cooling channels 20 arranged inside

  laughter of the metal hearth 11 is connected by one of its ends to a supply manifold 36, itself

  connected to a supply line 36 a, via

  diary of a non-return valve 38 The other end of the cooling channel 20 is connected to an evacuation manifold 37 itself connected to an evacuation pipe 37 a. The supply lines 36 a and 37 a are themselves connected to lines of a

  cooling such as lines 12 and 13 shown

seen in figure 1.

  In Figure 8, there is shown a check valve

  return 38 disposed at the inlet of a cooling channel

  formed inside the sole 11.

  The valve 38 has a chamber 39 inside

  the seat of which is welded 41 A ball of aluminum

  mine 40 comes to rest under the effect of its weight on the seat 41, so as to close the passage connecting the supply chamber 36 with the channel

  20, when the collector 36 is not supplied.

  When the collector is supplied, the ball 40 is raised by the stream of cooling water to its position 40 ′, so that water can enter the

cooling channel 20.

  il The cooling water circulating inside the cooling channel 20 is liable to

  spray at least partially at the start of cooling

  when natural circulation is not yet established, so that channel 20 is crossed by a

water-vapor emulsion.

  The valve 38 makes it possible to prevent the water-

  steam formed cannot escape through the supply manifold 36 This water-vapor emulsion can only escape through the outlet manifold 37 or hot manifold It therefore gradually establishes a natural circulation by difference in density of the two

  fluid columns which then keep the valve raised.

  In the case of a cooling device

  and protection according to the invention used in a reaction

  pressurized water nuclear reactor as currently constructed and operated, a sole is used whose surface substantially corresponds to the surface of the bottom of the well

of tank is 66 m 2.

  The metal sole made of solid steel

  truction A 42 has a thickness greater than 300 mm in its central part and has lateral walls intended to retain corium with a height close to

800 mm.

  The sole 11 is formed by juxtapo- blocks

  sés 30 a, 30 b, 31 a to 31 N whose length is close to 8 m. These blocks are machined so as to provide cooling channels whose diameter is close to

30 mm.

  The sole rests on the concrete slab 5 of which

  it provides protection at the bottom of the tank.

  The sole makes it possible to collect the molten corium, in the event of an incident resulting in a total cooling defect in the reactor, this

  corium having a volume greater than 40 m 3.

  The corium spilled on the metal sole 11 occupies a height greater than 600 mm in the central part of the sole 11, between the walls 43. The corium formed in the case of a core melting can be in the form of a continuous liquid medium or in the form of partially melted debris

  which pour onto the floor 11 of the cooling device

  smoothing, in the event of an accident In all cases, the corium spreads almost uniformly over the surface of the sole. In the case where the corium is a continuous molten medium, it is very fluid and a stable solidified crust cannot form on its surface, insofar as there is an intense release of heat beneath the surface of the corium. all of the highly fluid corium therefore spreads easily over the sole If the corium is made up of fragmented debris, it can form clumps of material located on the sole but these clumps cannot reach a size leading to a flow of heat higher than that appearing in the case of a

  uniform distribution of molten liquid corium.

  When the corium spreads over the metallic sole-

  that 11, it comes into contact with the structure 34 which absorbs part of the kinetic energy accompanying the fall of the corium The metal sole 11 is dimensioned to support the fall of the corium and in particular, the cooling channels 20 are spaced apart a distance P constituting the pitch of the cooling network, the value of which is such that the structure of the cooled hearth is equivalent to a massive structure capable of

  transfer important loads to raft 5

  hampering the fall of corium, without significant deformation of

cooling channels.

  When the corium whose temperature is

  near 24000 C comes into contact with the upper surface

  lower of the sole 11, the steel of the sole melts on a small thickness and mixes with the corium A thin crust (with a thickness of a few centimeters) is formed between the corium and the steel and a thermal equilibrium s 'establishes as soon as the energy absorbed by the cooling channels 20 becomes equal to the heat

released by the corium.

  In calculating the dimensions of the sole and the characteristics of the cooling system, it is assumed that the corium is completely dry and that it is therefore not cooled from above by evaporation of water. In addition, the heat flow radiated by the corium towards the tank well The circuit is designed to cool the corium, either in natural circulation or

in forced circulation.

  In steady state, when a solid crust

  solid formed on contact with the hearth, the temperature at the interface of the solidified crust and the upper surface of the hearth is lower than the melting temperature of the steel, i.e. 14,500 C All of the heat given off by the corium is evacuated by conduction in the

  solidified crust and in the metal sole.

  The power released by the corium collected by the hearth is around 32 MW, this power being

  evacuated by the cooling circuit.

  As soon as the corium comes into contact with the sole, the heating of this sole produces a partial boiling of the water, which leads to circulation

  natural if the pumps 17 are out of service.

  There is a natural circulation of water

  cooling in the channels with boiling through

  tielle; however, the channels are designed so that there can be no obstruction by steam. The natural circulation of the cooling water begins without any external intervention and the water passes through the

  pumps 17 of the cooling circuit.

  In the longer term, for example at the end of three days after the accident having caused the core to melt, the two pumps 17 of the cooling circuit can be restarted to establish forced circulation. We then obtain a continuous circulation

  leading to accelerated cooling and solidification

  fication of corium spilled on the sole.

  The pumps 17 can be driven by

thermal or electric motors.

  The circulating water heats up in the lower branch of each of the cooling channels before passing into the upper branch where it heats up while remaining liquid, in the case of forced circulation or by partially vaporizing, in the case

  of a natural circulation of a two-phase mixture.

  The device according to the invention is therefore extremely safe and allows effective protection of the reactor raft, insofar as the metal sole can withstand the fall of the corium without deforming, the circulation of the cooling fluid in the channels of the sole starting immediately after the fall of the

corium, passively.

  In addition, long-term cooling of the corium until it has completely solidified can be ensured

  easily by forced circulation of cooling water

is lying.

  The cooling of the corium is carried out without any vapor being produced inside the reactor building.

  The invention is not limited to the embodiment

tion which has been described.

  It is thus that one can imagine using metal soles produced in a different way from that which has been described. Instead of blocks welded to each other, one can use constituent elements of the sole which are assembled by interlocking and fixed to the raft by studs embedded and sealed in the raft and engaged in openings provided in each of

elements of the sole.

  The sole can have variable shapes and dimensions, depending on the characteristics of the tank and the

reactor vessel wells.

  The cooling circuit of the hearth can be realized in a different way from that which has been described. It is however preferable to have the active elements of the cooling circuit such as the heat exchangers outside the enclosure of

reactor safety.

  Forced circulation of the coolant

  This can be ensured by any type of circulation pump.

  tion generally used in cooling circuits

  development of a nuclear reactor.

  Finally, the cooling and protection device according to the invention can be used in the

  case of any nuclear reactor comprising a vessel containing

  mant the reactor core placed in a vessel well, the bottom of which is made up of part of the structure

  nuclear reactor such as a raft.

Claims (10)

  1 device for cooling the core (4) and for protecting the concrete structure (5) of a nuclear reactor, the core (4) of which has melted following an accident, the reactor comprising a vessel (3 ) of generally cylindrical shape enclosing the core (4) of the reactor arranged with its vertical axis in a vessel well (2) having a lower bottom on which the cooling and protection device (11) rests, constituted by a part (5 ) of the reactor structure comprising a part of the raft located directly above the tank (3), characterized in that it is constituted by a metal sole (11) covering the bottom of the well   tank in which cooling channels are provided   ment (20) by circulation of a fluid connected to means for supplying and discharging (12, 13, 14, 15) of the fluid cooling.   2. Device according to claim 1, characterized in that the slab (11) consists of blocks (30 a, 30 b, 31 a, 31 b ,, 31 n) attached and welded to each other along the faces vertical sides. 3. Device according to claim 2, characterized in that the cooling channels (20) are constituted by machined parts of the faces reported against each other of the blocks (30 a, 30 b, 31 a ,, 31 n).   4. Device according to any one of   Claims 2 and 3, characterized in that the sole   metal (11) has two end blocks (30 a, 30 b)   at its longitudinal ends and intermediate blocks   res (31 a, 31 n) between the end blocks (30 a, 30 b), the end blocks (30 a, 30 b) having upper parts (33) projecting from the upper surface of the intermediate blocks (31 a, 31 n) constituting   a low wall intended to contain the molten core of the reaction   tor, during a spillage of this heart on the sole (11).   5. Device according to any one of   Claims 1 to 4, characterized in that the sole   (11) comprises a metal sheet structure resting on its horizontal upper surface with a certain spacing and intended to absorb the kinetic energy of the molten core of the reactor, during a spill on the sole (11).   6 Device according to claim 4, characterized in that the projecting parts (43)   end blocks (30 a, 30 b) of the sole (11) comprising   try facing internal faces topped with a layer of refractory material (35).   7 Device according to any one of   Claims 1 to 6, characterized in that the   means for supplying and discharging the coolant   dissement of the sole (11) comprise collectors (24,, 36, 37) communicating with the end parts of the   cooling channels (20), supply lines   tion and discharge of the cooling fluid (21 a, 21 b, 22 a, 22 b, 36 a, 37 a) and a cooling circuit comprising at least one heat exchanger (14) and   pipes (12, 13) connected to the supply pipes   supply and discharge collectors   sole cooling fluid.   8. Device according to claim 7, characterized in that the cooling circuit is disposed at least partially outside a safety enclosure (1) constituting a part of the reactor structure inside which is   arranged the tank (3) enclosing the heart (4).   9. Device according to claim 8, characterized in that the cooling circuit comprises at least one heat exchanger (14), at least one expansion and condensation tank (16) and a circulation pump (17) arranged in series, outside the safety enclosure (1), on pipes (12, 13) of the cooling circuit.   10 Device according to any one of   Claims 1 to 9, characterized in that a valve   non-return valve (38) is arranged on the end part of each of the cooling channels connected to a means   supply (36) of cooling fluid.